1. Field of the Invention
The present invention relates to the field of magnetic pump drives and, more particularly, to a magnetic pump drive comprising a double-walled isolating shell. In particular, the magnet pump drive comprises a driver driven by a motor, a rotor encircled by the driver and connected with an impeller of the pump, at least two circles of permanent magnets, one of the two circles being attached to a cylindrical face of the rotor respectively and the driver in opposite relation and a stationary isolation shell. The stationary isolation shell, which encircles the rotor, is filled by the medium to be pumped, and is sealed against the housing of the pump. The force of the permanent magnets passes through the shell for effecting the drive from the driver to the rotor.
2. Descripton of the Prior Art
Magnetic pump drives have been known in the art for many years. The known drives have proven to be reliable in the chemical industry and in nuclear power stations. Because there is no shaft duct and thus no leakage along a shaft out of a sealing, these pumps have only gaskets which are more easily handled and which are much less prone to a leak than a seal against a rotating shaft.
In order to achieve a satisfying efficiency, the permanent magnets which are arranged on opposite sides on the rotor and the driver respectively and which are arranged on each device in changing polarity for a permanent pulling force, have to be close together. However, tolerances and especially safety against damage to the isolation shell has to be observed. Generally, there is a distance of approximately 4 mm between two permanent magnets which face each other and which are attached to the rotor and the driver respectively. The gap between the two circles of permanent magnets is crossed by the cylindrical section of the isolation shell, and this section is generally 1 to 2 mm thick.
As a result of damage or wear to the bearings of the driver and/or the rotor as well as in the case of an inadvertent introduction of a solid particle in the medium to be pumped, it may occur that the isolation shell is damaged so that the medium contaminates the environment which was retained by the isolation shell. It is common practice to use alarm devices and also to maintain a sealing of the driver housing which is capable of retaining the medium in the event of a damaged isolation shell even after an extened dry running.
Recently, the effectiveness of transmitting the motor performance onto the impeller of a pump by using a permanent magnet coupling has become of reduced importance. Improved safety is much more appreciated which is mainly achieved by thickening the wall sections of the isolation shell. Consequently, not only the gap between each pair of opposed permanent magnets is increased but also the thermal loss by eddy currents within the isolation shell unless the isolation shell is constructed from electrically non-conductive material as, for instance, plastic materials.